RTP Payload Formats for European Telecommunications Standards Institute (ETSI) European Standard ES 202 050, ES 202 211, and ES 202 212 Distributed Speech Recognition Encoding
draft-ietf-avt-rtp-dsr-codecs-03

Audio Video Transport WG                                          Q. Xie
Internet-Draft                                                 D. Pearce
Expires: December 16, 2004                                      Motorola
                                                            June 17, 2004

      RTP Payload Formats for European Telecommunications Standards
  Institute (ETSI) European Standard ES 202 050, ES 202 211, and ES 202
               212 Distributed Speech Recognition Encoding
                   draft-ietf-avt-rtp-dsr-codecs-03.txt

Status of this Memo

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Abstract

    This document specifies RTP payload formats for encapsulating ETSI
    Standard ES 202 050 DSR Advanced Front-end (AFE), ES 202 211 DSR
    Extended Front-end (XFE), and ES 202 212 DSR Extended Advanced
    Front-end (XAFE) signal processing feature streams for distributed
    speech recognition (DSR) systems.

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Table of Contents

    1.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
    2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
      2.1   ETSI ES 202 050 Advanced DSR Front-end Codec . . . . . . .  4
      2.2   ETSI ES 202 211 Extended DSR Front-end Codec . . . . . . .  4
      2.3   ETSI ES 202 212 Extended Advanced DSR Front-end Codec  . .  5
    3.  DSR RTP Payload Formats  . . . . . . . . . . . . . . . . . . .  6
      3.1   Common Considerations of the Three DSR RTP Payload
            Formats  . . . . . . . . . . . . . . . . . . . . . . . . .  6
        3.1.1   Number of FPs in Each RTP Packet . . . . . . . . . . .  6
        3.1.2   Support for Discontinuous Transmission . . . . . . . .  6
        3.1.3   RTP header usage . . . . . . . . . . . . . . . . . . .  6
      3.2   Payload Format for ES 202 050 DSR  . . . . . . . . . . . .  7
        3.2.1   Frame Pair Formats . . . . . . . . . . . . . . . . . .  7
      3.3   Payload Format for ES 202 211 DSR  . . . . . . . . . . . .  9
        3.3.1   Frame Pair Formats . . . . . . . . . . . . . . . . . .  9
      3.4   Payload Format ES 202 212 DSR  . . . . . . . . . . . . . . 11
        3.4.1   Frame Pair Formats . . . . . . . . . . . . . . . . . . 11
    4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
      4.1   Mapping MIME Parameters into SDP . . . . . . . . . . . . . 15
      4.2   Usage in Offer/Answer  . . . . . . . . . . . . . . . . . . 16
    5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
    6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
    7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
    7.1   Normative References . . . . . . . . . . . . . . . . . . . . 16
    7.2   Informative References . . . . . . . . . . . . . . . . . . . 17
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17
        Intellectual Property and Copyright Statements . . . . . . . . 19

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1.  Conventions

    The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
    SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
    they appear in this document, are to be interpreted as described in
    RFC 2119 [5].

    The following acronyms are used in this document:

       DSR  - Distributed Speech Recognition
       ETSI - the European Telecommunications Standards Institute
       FP   - Frame Pair
       DTX  - Discontinuous Transmission
       VAD  - Voice Activity Detection

2.  Introduction

    Distributed speech recognition (DSR) technology is intended for a
    remote device acting as a thin client, also known as the front-end,
    to communicate with a speech recognition server, also called a speech
    engine, over a network connection to obtain speech recognition
    services.  More details on DSR over Internet can be found in RFC 3557
    [11].

    To achieve interoperability with different client devices and speech
    engines, the first ETSI standard DSR front-end ES 201 108 was
    published in early 2000 [12], and an RTP packetization for ES 201 108
    frames is defined in RFC 3557 [11] by IETF.

    In ES 202 050 [1], ETSI issues another standard for an Advanced DSR
    front-end that provides substantially improved recognition
    performance when background noise is present.  The codecs in ES 202
    050 uses a slightly different frame format from that of ES 201 108
    and thus the two do not inter-operate with each other.

    The RTP packetization for ES 202 050 front-end defined in this
    document uses the same RTP packet format layout as that defined in
    RFC 3557 [11].  The differences are in the DSR codec frame bit
    definition and the payload type MIME registration.

    The two further standards, ES 202 211 and ES 202 212, provided
    extensions to each of the DSR front-end standards.  The extensions
    allow the speech waveform to be reconstructed for human audition and
    can also be used to improve recognition performance for tonal
    languages.  This is done by sending additional pitch and voicing
    information for each frame along with the recognition features.

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    The RTP packet format for these extended standards are also defined
    in this document.

    It is worthwhile to note that the performance of most speech
    recognizers are extremely sensitive to consecutive frame losses and
    the DSR speech recognizers are no exception.  If a DSR over RTP
    session is expected to endure high packet loss ratio between the
    front-end and the speech engine, one should consider limiting the
    maximum number of DSR frames allowed in a packet, or employing other
    loss management techniques, such as FEC or interleaving, to minimize
    the chance of losing consecutive frames.

2.1  ETSI ES 202 050 Advanced DSR Front-end Codec

    Some relevant characteristics of ES 202 050 Advanced DSR front-end
    codec are summarized below.

    The front-end calculation is a frame-based scheme that produces an
    output vector every 10 ms.  In the front-end feature extraction,
    noise reduction by two stages of Wiener filtering is performed first.
    Then, waveform processing is applied to the de-noised signal and
    mel-cepstral features are calculated.  At the end, blind equalization
    is applied to the cepstral features.  The front-end algorithm
    produces at its output a mel-cepstral representation in the same
    format as ES 210 108, i.e., 12 cepstral coeffients [C1 - C12], C0 and
    log Energy.  Voice activity detection (VAD) for the classification of
    each frame as speech or non-speech is also implemented in Feature
    Extraction.  The VAD information is included in the payload format
    for each frame pair to be sent to the remote recognition engine as
    part of the payload.  This information may optionally be used by the
    receiving recognition engine to drop non-speech frames.  The
    front-end supports three raw sampling rates: 8 kHz, 11 kHz, and 16
    kHz (It is worthwhile to note that unlike some other speech codecs,
    the feature frame size of DSR presented to RTP packetization is not
    dependent on the number of speech samples used in each 10 ms sample
    frame.  This will become more evident in the following sections).

    After calculation of the mel-cepstral representation, the
    representation is first quantized via split-vector quantization to
    reduce the data rate of the encoded stream.  Then, the quantized
    vectors from two consecutive frames are put into an frame pair (FP),
    as described in more detail in Section 3.2 below.

2.2  ETSI ES 202 211 Extended DSR Front-end Codec

    Some relevant characteristics of ES 202 211 Extended DSR front-end
    codec are summarized below.

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    ES 202 211 is an extension of the mel-cepstrum DSR Front-end standard
    ES 201 108 [12].  The mel-cepstrum front-end provides the features
    for speech recognition but these are not available for human
    listening.  The purpose of the extension is allow the reconstruction
    of the speech waveform from these features so that they can be
    replayed.  The front-end feature extraction part of the processing is
    exactly the same as for ES 201 108.  To allow speech reconstruction
    additional fundamental frequency (perceived as pitch) and voicing
    class (e.g.  non-speech, voiced, unvoiced and mixed) information is
    needed.  This is the extra information that is provided by the
    extended front-end processing algorithms at the device side that is
    compressed and transmitted along with the front-end features to the
    server.  This extra information may also be useful for improved
    speech recognition performance with tonal languages such as Mandarin,
    Cantonese and Thai.

    Full information about the client side signal processing algorithms
    used in the standard are described in the specification ES 202 211
    [2].

    The additional fundamental frequency and voicing class information is
    compressed for each frame pair.  The pitch for the first frame of the
    FP is quantised to 7 bits and the second frame is differentially
    quantized with 5 bits.  The voicing class is indicated with one bit
    for each frame.  The total for the extension information for a frame
    pair therefore consists of 14 bits plus and additional 2 bits of CRC
    error protection computed over these extension bits only.

    The total information for the frame pair is made up of 92 bits for
    the two compressed front-end feature frames (including 4 bits for
    their CRC) plus 16 bits for the extension (including 2 bits for their
    CRC) and 4 bits of null padding to give a total of 14 octets per
    frame pair.  As for ES 201 208 the extended frame pair also
    corresponds to 20ms of speech.  The extended front-end supports three
    raw sampling rates: 8 kHz, 11 kHz, and 16 kHz.

    The quantized vectors from two consecutive frames are put into an FP,
    as described in more detail in Section 3.3 below.

    The parameters received at the remote server from the RTP extended
    DSR payload specified here can be used to synthesize an intelligible
    speech waveform for replay.  The algorithms to do this are described
    in the specification ES 202 211 [2].

2.3  ETSI ES 202 212 Extended Advanced DSR Front-end Codec

    ES 202 212 is the extension for the DSR Advanced Front-end ES 202 050
    [1].  It provides the same capabilities as the extended mel-cepstrum

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    front-end described in section 2.2 but for the DSR Advanced
    Front-end.

3.  DSR RTP Payload Formats

3.1  Common Considerations of the Three DSR RTP Payload Formats

    The three DSR RTP payload formats defined in this document share the
    following consideration or behaviours.

3.1.1  Number of FPs in Each RTP Packet

    Any number of FPs MAY be aggregate together in an RTP payload and
    they MUST be consecutive in time.  However, one SHOULD always keep
    the RTP payload size smaller than the MTU in order to avoid IP
    fragmentation and SHOULD follow the recommendations given in Section
    3.1 in RFC 3557 [11] when determining the proper number of FPs in an
    RTP payload.

3.1.2  Support for Discontinuous Transmission

    Same considerations described in Section 3.2 of RFC 3557 [11] apply
    to all the three DSR RTP payloads defined in this document.

3.1.3  RTP header usage

    The format of the RTP header is specified in RFC 3550 [9].  The three
    payload formats defined here use the fields of the header in a manner
    consistent with that specification.

    The RTP timestamp corresponds to the sampling instant of the first
    sample encoded for the first FP in the packet.  The timestamp clock
    frequency is the same as the sampling frequency, so the timestamp
    unit is in samples.

    As defined by all the three front-end codecs, the duration of one FP
    is 20 ms, corresponding to 160, 220, or 320 encoded samples with
    sampling rate of 8, 11, or 16 kHz being used at the front-end,
    respectively.  Thus, the timestamp is increased by 160, 220, or 320
    for each consecutive FP, respectively.

    The DSR payload for all these three front-end codecs is always an
    integral number of octets.  If additional padding is required for
    some other purpose, then the P bit in the RTP in the header may be
    set and padding appended as specified in RFC 3550 [9].

    The RTP header marker bit (M) MUST be set following the general rules
    for audio codecs as defined in Section 4.1 in RFC 3551 [10].

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    The assignment of an RTP payload type for these three new packet
    formats is outside the scope of this document, and will not be
    specified here.  It is expected that the RTP profile under which any
    of these payload formats is being used will assign a payload type for
    this encoding or specify that the payload type is to be bound
    dynamically.

3.2  Payload Format for ES 202 050 DSR

    An ES 202 050 DSR RTP payload datagram uses exactly the same layout
    as defined in Section 3 of RFC 3557 [11], i.e., a standard RTP header
    followed by a DSR payload containing a series of DSR FPs.

    The size of each ES 202 050 FP is still 96 bits or 12 octets (defined
    in the following sections).  This ensures that a DSR RTP payload will
    always end on an octet boundary.

3.2.1  Frame Pair Formats

3.2.1.1  Format of Speech and Non-speech FPs

    The following mel-cepstral frame MUST be used, as defined in [1]:

    As defined in [1], pairs of the quantized 10ms mel-cepstral frames
    MUST be grouped together and protected with a 4-bit CRC, forming a
    92-bit long FP.  At the end, each FP MUST be padded with 4 zeros to
    the MSB 4 bits of the last octet in order to make the FP aligned to
    the octet boundary.

    The following diagram shows a complete ES 202 050 FP:

      Frame #1 in FP:
      ===============
         (MSB)                                     (LSB)
           0     1     2     3     4     5     6     7
        +-----+-----+-----+-----+-----+-----+-----+-----+
        :  idx(2,3) |            idx(0,1)               |    Octet 1
        +-----+-----+-----+-----+-----+-----+-----+-----+
        :       idx(4,5)        |     idx(2,3) (cont)   :    Octet 2
        +-----+-----+-----+-----+-----+-----+-----+-----+
        |             idx(6,7)              |idx(4,5)(cont)  Octet 3
        +-----+-----+-----+-----+-----+-----+-----+-----+
    idx(10,11)| VAD |              idx(8,9)             |    Octet 4
        +-----+-----+-----+-----+-----+-----+-----+-----+
        :       idx(12,13)      |   idx(10,11) (cont)   :    Octet 5
        +-----+-----+-----+-----+-----+-----+-----+-----+
                                |   idx(12,13) (cont)   :    Octet 6/1
                                +-----+-----+-----+-----+

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     Frame #2 in FP:
     ===============
         (MSB)                                     (LSB)
           0     1     2     3     4     5     6     7
        +-----+-----+-----+-----+
        :        idx(0,1)       |                            Octet 6/2
        +-----+-----+-----+-----+-----+-----+-----+-----+
        |              idx(2,3)             |idx(0,1)(cont)  Octet 7
        +-----+-----+-----+-----+-----+-----+-----+-----+
        :  idx(6,7) |              idx(4,5)             |    Octet 8
        +-----+-----+-----+-----+-----+-----+-----+-----+
        :        idx(8,9)       |      idx(6,7) (cont)  :    Octet 9
        +-----+-----+-----+-----+-----+-----+-----+-----+
        |          idx(10,11)         | VAD |idx(8,9)(cont)  Octet 10
        +-----+-----+-----+-----+-----+-----+-----+-----+
        |                   idx(12,13)                  |    Octet 11
        +-----+-----+-----+-----+-----+-----+-----+-----+

     CRC for Frame #1 and Frame #2 and padding in FP:
     ================================================
         (MSB)                                     (LSB)
           0     1     2     3     4     5     6     7
        +-----+-----+-----+-----+-----+-----+-----+-----+
        |  0  |  0  |  0  |  0  |          CRC          |    Octet 12
        +-----+-----+-----+-----+-----+-----+-----+-----+

    The 4-bit CRC in the FP MUST be calculated using the formula
    (including the bit-order rules) defined in 7.2 in [1].

    Therefore, each FP represents 20ms of original speech.  Note, as
    shown above, each FP MUST be padded with 4 zeros to the MSB 4 bits of
    the last octet in order to make the FP aligned to the octet boundary.
    This makes the total size of an FP 96 bits, or 12 octets.  Note, this
    padding is separate from padding indicated by the P bit in the RTP
    header.

    The definition of the indices and 'VAD' flag are described in [1] and
    their value is only set and examined by the codecs in the front-end
    client and the recognizer.

3.2.1.2  Format of Null FP

    Null FPs are sent to mark the end of a transmission segment.  Details
    on transmission segment and the use of Null FPs can be found in RFC
    3557 [11].

    A Null FP for the ES 202 050 front-end codec is defined by setting

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    the content of the first and second frame in the FP to null (i.e.,
    filling the first 88 bits of the FP with 0's).  The 4-bit CRC MUST be
    calculated the same way as described in 7.2.4 in [1], and 4 zeros
    MUST be padded to the end of the Null FP to made it octet aligned.

3.3  Payload Format for ES 202 211 DSR

    An ES 202 211 DSR RTP payload datagram is very similar to that
    defined in Section 3 of RFC 3557 [11], i.e., a standard RTP header
    followed by a DSR payload containing a series of DSR FPs.

    The size of each ES 202 211 FP is 112 bits or 14 octets (defined in
    the following sections).  This ensures that a DSR RTP payload will
    always end on an octet boundary.

3.3.1  Frame Pair Formats

3.3.1.1  Format of Speech and Non-speech FPs

    The following mel-cepstral frame MUST be used, as defined in Section
    6.2.4 in [2]:

    As defined in Section 6.2.4 in [2], after two frames (Frame #1 and
    Frame #2) worth of codebook indices, or 88 bits, a 4-bit CRC
    calculated on these 88 bits immediately follows it.  The pitch
    indices of the first frame (Pidx1: 7 bits) and the second frame
    (Pidx2: 5 bits) of the frame pair then follow.  The class indices of
    the two frames in the frame pair worth 1 bit each (Cidx1 and Cidx2)
    next follow.  Finally, a 2-bit CRC calculated on the pitch and class
    bits (total: 14 bits) of the frame pair is included (PC-CRC).  The
    total number of bits in frame pair packet is therefore 44 + 44 + 4 +
    7 + 5 + 1 + 1 + 2 = 108.  At the end, each FP MUST be padded with 4
    zeros to the MSB 4 bits of the last octet in order to make the FP
    aligned to the octet boundary.

    The following diagram shows a complete ES 202 211 FP:

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      Frame #1 in FP:
      ===============
        (MSB)                                     (LSB)
          0     1     2     3     4     5     6     7
       +-----+-----+-----+-----+-----+-----+-----+-----+
       :  idx(2,3) |            idx(0,1)               |    Octet 1
       +-----+-----+-----+-----+-----+-----+-----+-----+
       :       idx(4,5)        |     idx(2,3) (cont)   :    Octet 2
       +-----+-----+-----+-----+-----+-----+-----+-----+
       |             idx(6,7)              |idx(4,5)(cont)  Octet 3
       +-----+-----+-----+-----+-----+-----+-----+-----+
        idx(10,11) |              idx(8,9)             |    Octet 4
       +-----+-----+-----+-----+-----+-----+-----+-----+
       :       idx(12,13)      |   idx(10,11) (cont)   :    Octet 5
       +-----+-----+-----+-----+-----+-----+-----+-----+
                               |   idx(12,13) (cont)   :    Octet 6/1
                               +-----+-----+-----+-----+

     Frame #2 in FP:
     ===============
        (MSB)                                     (LSB)
          0     1     2     3     4     5     6     7
       +-----+-----+-----+-----+
       :        idx(0,1)       |                            Octet 6/2
       +-----+-----+-----+-----+-----+-----+-----+-----+
       |              idx(2,3)             |idx(0,1)(cont)  Octet 7
       +-----+-----+-----+-----+-----+-----+-----+-----+
       :  idx(6,7) |              idx(4,5)             |    Octet 8
       +-----+-----+-----+-----+-----+-----+-----+-----+
       :        idx(8,9)       |      idx(6,7) (cont)  :    Octet 9
       +-----+-----+-----+-----+-----+-----+-----+-----+
       |          idx(10,11)               |idx(8,9)(cont)  Octet 10
       +-----+-----+-----+-----+-----+-----+-----+-----+
       |                   idx(12,13)                  |    Octet 11
       +-----+-----+-----+-----+-----+-----+-----+-----+

     CRC for Frame #1 and Frame #2 in FP:
     ====================================
        (MSB)                                     (LSB)
          0     1     2     3     4     5     6     7
                               +-----+-----+-----+-----+
                               |          CRC          |    Octet 12/1
                               +-----+-----+-----+-----+

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     Extension information and padding in FP:
     ========================================
        (MSB)                                     (LSB)
          0     1     2     3     4     5     6     7
       +-----+-----+-----+-----+
       :       Pidx1           |                            Octet 12/2
       +-----+-----+-----+-----+-----+-----+-----+-----+
       |            Pidx2            |   Pidx1 (cont)  :    Octet 13
       +-----+-----+-----+-----+-----+-----+-----+-----+
       |  0  |  0  |  0  |  0  |  PC-CRC   |Cidx2|Cidx1|    Octet 14
       +-----+-----+-----+-----+-----+-----+-----+-----+

    The 4-bit CRC and the 2-bit PC-CRC in the FP MUST be calculated using
    the formula (including the bit-order rules) defined in 6.2.4 in [2].

    Therefore, each FP represents 20ms of original speech.  Note, as
    shown above, each FP MUST be padded with 4 zeros to the MSB 4 bits of
    the last octet in order to make the FP aligned to the octet boundary.
    This makes the total size of an FP 112 bits, or 14 octets.  Note,
    this padding is separate from padding indicated by the P bit in the
    RTP header.

3.3.1.2  Format of Null FP

    A Null FP for the ES 202 211 front-end codec is defined by setting
    all the 112 bits of the FP with 0's.  Null FPs are sent to mark the
    end of a transmission segment.  Details on transmission segment and
    the use of Null FPs can be found in RFC 3557 [11].

3.4  Payload Format ES 202 212 DSR

    Similar to other ETSI DSR front-end encoding schemes, the encoded DSR
    feature stream of ES 202 212 is transmitted in a sequence of frame
    pairs (FPs), where each FP represents two consecutive original voice
    frames.

    An ES 202 212 DSR RTP payload datagram is very similar to that
    defined in Section 3 of RFC 3557 [11], i.e., a standard RTP header
    followed by a DSR payload containing a series of DSR FPs.

    The size of each ES 202 212 FP is 112 bits or 14 octets (defined in
    the following sections).  This ensures that an ES 202 212 DSR RTP
    payload will always end on an octet boundary.

3.4.1  Frame Pair Formats

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3.4.1.1  Format of Speech and Non-speech FPs

    The following mel-cepstral frame MUST be used, as defined in Section
    7.2.4 in [3]:

    As defined in Section 7.2.4 in [3], after two frames (Frame #1 and
    Frame #2) worth of codebook indices, or 88 bits, a 4-bit CRC
    calculated on these 88 bits immediately follows it.  The pitch
    indices of the first frame (Pidx1: 7 bits) and the second frame
    (Pidx2: 5 bits) of the frame pair then follow.  The class indices of
    the two frames in the frame pair worth 1 bit each next follow (Cidx1
    and Cidx2).  Finally, a 2-bit CRC (PC-CRC) calculated on the pitch
    and class bits (total: 14 bits) of the frame pair is included.  The
    total number of bits in frame pair packet is therefore 44 + 44 + 4 +
    7 + 5 + 1 + 1 + 2 = 108.  At the end, each FP MUST be padded with 4
    zeros to the MSB 4 bits of the last octet in order to make the FP
    aligned to the octet boundary.  The padding brings the total size of
    a FP to 112 bits, or 14 octets.  Note, this padding is separate from
    padding indicated by the P bit in the RTP header.

    The following diagram shows a complete ES 202 212 FP:

      Frame #1 in FP:
      ===============
         (MSB)                                     (LSB)
           0     1     2     3     4     5     6     7
        +-----+-----+-----+-----+-----+-----+-----+-----+
        :  idx(2,3) |            idx(0,1)               |    Octet 1
        +-----+-----+-----+-----+-----+-----+-----+-----+
        :       idx(4,5)        |     idx(2,3) (cont)   :    Octet 2
        +-----+-----+-----+-----+-----+-----+-----+-----+
        |             idx(6,7)              |idx(4,5)(cont)  Octet 3
        +-----+-----+-----+-----+-----+-----+-----+-----+
    idx(10,11)| VAD |              idx(8,9)             |    Octet 4
        +-----+-----+-----+-----+-----+-----+-----+-----+
        :       idx(12,13)      |   idx(10,11) (cont)   :    Octet 5
        +-----+-----+-----+-----+-----+-----+-----+-----+
                                |   idx(12,13) (cont)   :    Octet 6/1
                                +-----+-----+-----+-----+

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     Frame #2 in FP:
     ===============
         (MSB)                                     (LSB)
           0     1     2     3     4     5     6     7
        +-----+-----+-----+-----+
        :        idx(0,1)       |                            Octet 6/2
        +-----+-----+-----+-----+-----+-----+-----+-----+
        |              idx(2,3)             |idx(0,1)(cont)  Octet 7
        +-----+-----+-----+-----+-----+-----+-----+-----+
        :  idx(6,7) |              idx(4,5)             |    Octet 8
        +-----+-----+-----+-----+-----+-----+-----+-----+
        :        idx(8,9)       |      idx(6,7) (cont)  :    Octet 9
        +-----+-----+-----+-----+-----+-----+-----+-----+
        |          idx(10,11)         | VAD |idx(8,9)(cont)  Octet 10
        +-----+-----+-----+-----+-----+-----+-----+-----+
        |                   idx(12,13)                  |    Octet 11
        +-----+-----+-----+-----+-----+-----+-----+-----+

     CRC for Frame #1 and Frame #2 in FP:
     ====================================
         (MSB)                                     (LSB)
           0     1     2     3     4     5     6     7
                                +-----+-----+-----+-----+
                                |          CRC          |    Octet 12/1
                                +-----+-----+-----+-----+

     Extension information and padding in FP:
     ========================================
         (MSB)                                     (LSB)
           0     1     2     3     4     5     6     7
        +-----+-----+-----+-----+
        :       Pidx1           |                            Octet 12/2
        +-----+-----+-----+-----+-----+-----+-----+-----+
        |            Pidx2            |   Pidx1 (cont)  :    Octet 13
        +-----+-----+-----+-----+-----+-----+-----+-----+
        |  0  |  0  |  0  |  0  |  PC-CRC   |Cidx2|Cidx1|    Octet 14
        +-----+-----+-----+-----+-----+-----+-----+-----+

    The codebook indices, VAD flag, pitch index, and class index are
    specified in Section 6 of [3].  The 4-bit CRC and the 2-bit PC-CRC in
    the FP MUST be calculated using the formula (including the bit-order
    rules) defined in 7.2.4 in [3].

3.4.1.2  Format of Null FP

    A Null FP for the ES 202 212 front-end codec is defined by setting

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    all the 112 bits of the FP with 0's.  Null FPs are sent to mark the
    end of a transmission segment.  Details on transmission segment and
    the use of Null FPs can be found in RFC 3557 [11].

4.  IANA Considerations

    For each of the three ETSI DSR front-end codecs covered in this
    document, a new MIME subtype registration is required for the
    corresponding payload type, as described below.

    Media Type name: audio

    Media subtype names:

          dsr-es202050 (for ES 202 050 front-end)

          dsr-es202211 (for ES 202 211 front-end)

          dsr-es202212 (for ES 202 212 front-end)

    Required parameters: none

    Optional parameters:

    rate: Indicates the sample rate of the speech.  Valid values include:
       8000, 11000, and 16000.  If this parameter is not present, 8000
       sample rate is assumed.

    maxptime: see RFC 3267 [8].  If this parameter is not present,
       maxptime is assumed to be 80ms.

       Note, since the performance of most speech recognizers are
       extremely sensitive to consecutive FP losses, if the user of the
       payload format expects a high packet loss ratio for the session,
       it MAY consider to explicitly choose a maxptime value for the
       session that is shorter than the default value.

    ptime: see RFC 2327 [6].

    Encoding considerations: These types are defined for transfer via RTP
       [9] as described in Section 3 of RFC XXXX.

    Security considerations: See Section 5 of RFC XXXX.

    Person & email address to contact for further information:
       Qiaobing.Xie@motorola.com

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    Intended usage: COMMON.  It is expected that many VoIP applications
       (as well as mobile applications) will use this type.

    Author/Change controller:

       *  Qiaobing.Xie@motorola.com

       *  IETF Audio/Video transport working group

4.1  Mapping MIME Parameters into SDP

    The information carried in the MIME media type specification has a
    specific mapping to fields in the Session Description Protocol (SDP)
    [6], which is commonly used to describe RTP sessions.  When SDP is
    used to specify sessions employing ES 202 050, ES 202 211, or ES 202
    212 DSR codec, the mapping is as follows:

    o  The MIME type ("audio") goes in SDP "m=" as the media name.

    o  The MIME subtype ("dsr-es202050", "dsr-es202211", or
       "dsr-es202212") goes in SDP "a=rtpmap" as the encoding name.

    o  The optional parameter "rate" also goes in "a=rtpmap" as clock
       rate.  If no rate is given, then the default value (i.e., 8000) is
       used in SDP.

    o  The optional parameters "ptime" and "maxptime" go in the SDP
       "a=ptime" and "a=maxptime" attributes, respectively.

    Example of usage of ES 202 050 DSR:

      m=audio 49120 RTP/AVP 101
      a=rtpmap:101 dsr-es202050/8000
      a=maxptime:40

    Example of usage of ES 202 211 DSR:

      m=audio 49120 RTP/AVP 101
      a=rtpmap:101 dsr-es202211/8000
      a=maxptime:40

    Example of usage of ES 202 212 DSR:

      m=audio 49120 RTP/AVP 101
      a=rtpmap:101 dsr-es202212/8000
      a=maxptime:40

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4.2  Usage in Offer/Answer

    All SDP parameters in this payload format are declarative, and all
    reasonable values are expected to be supported.  Thus, the standard
    usage of Offer/Answer as described in RFC 3264 [7] should be
    followed.

5.  Security Considerations

    Implementations using the payload defined in this specification are
    subject to the security considerations discussed in the RTP
    specification RFC 3550 [9] and any RTP profile, e.g.  RFC 3551 [10].
    This payload does not specify any different security services.

    Congestion control for RTP MUST be used in accordance with RFC 3550
    [9], and any applicable RTP profile, e.g.  RFC 3551 [10].

6.  Acknowledgments

    The design presented here is based on that of RFC 3557 [11].  The
    authors wish to thank for the review and comments from Magnus
    Westerlund and others.

7.  References

7.1  Normative References

    [1]   European Telecommunications Standards Institute (ETSI) Standard
          ES 202 050, "Speech Processing, Transmission and Quality
          Aspects (STQ); Distributed Speech Recognition; Front-end
          Feature Extraction Algorithm; Compression Algorithms", (http://
          pda.etsi.org/pda/) , October 2002.

    [2]   European Telecommunications Standards Institute (ETSI) Standard
          ES 202 211, "Speech Processing, Transmission and Quality
          Aspects (STQ); Distributed Speech Recognition; Extended
          front-end feature extraction algorithm; Compression algorithms;
          Back-end speech reconstruction algorithm",
          (http://pda.etsi.org/pda/) , November 2003.

    [3]   European Telecommunications Standards Institute (ETSI) Standard
          ES 202 212, "Speech Processing, Transmission and Quality
          aspects (STQ); Distributed speech recognition; Extended
          advanced front-end feature extraction algorithm; Compression
          algorithms; Back-end speech reconstruction algorithm", (http://
          pda.etsi.org/pda/) , November 2003.

    [4]   Bradner, S., "The Internet Standards Process -- Revision 3",

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          BCP 9, RFC 2026, October 1996.

    [5]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
          Levels", BCP 14, RFC 2119, March 1997.

    [6]   Handley, M. and V. Jacobson, "SDP: Session Description
          Protocol", RFC 2327, April 1998.

    [7]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
          the Session Description Protocol (SDP)", RFC 3264, June 2002.

    [8]   Sjoberg, J., Westerlund, M., Lakaniemi, A. and Q. Xie,
          "Real-Time Transport Protocol (RTP) Payload Format and File
          Storage Format for the Adaptive Multi-Rate (AMR) and Adaptive
          Multi-Rate Wideband (AMR-WB) Audio Codecs", RFC 3267, June
          2002.

    [9]   Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
          "RTP: A Transport Protocol for Real-Time Applications", RFC
          3550, July 2003.

    [10]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
          Conferences with Minimal Control", RFC 3551, July 2003.

    [11]  Xie, Q., "RTP Payload Format for European Telecommunications
          Standards Institute (ETSI) European Standard ES 201 108
          Distributed Speech Recognition Encoding", RFC 3557, July 2003.

7.2  Informative References

    [12]  European Telecommunications Standards Institute (ETSI) Standard
          ES 201 108, "Speech Processing, Transmission and Quality
          Aspects (STQ); Distributed Speech Recognition; Front-end
          Feature Extraction Algorithm; Compression Algorithms", (http://
          webapp.etsi.org/pda/) , April 2000.

Authors' Addresses

    Qiaobing Xie
    Motorola, Inc.
    1501 W. Shure Drive, 2-F9
    Arlington Heights, IL  60004
    US

    Phone: +1-847-632-3028
    EMail: qxie1@email.mot.com

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    David Pearce
    Motorola Labs
    UK Research Laboratory
    Jays Close
    Viables Industrial Estate
    Basingstoke, HANTS  RG22 4PD
    UK

    Phone: +44 (0)1256 484 436
    EMail: bdp003@motorola.com

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